Butterfly valve

ABSTRACT

The butterfly valve includes a body having a generally cylindrical bore defining a flow passage and a generally circular, stem-mounted disc member rotatably mounted within the bore for shutting off flow. The disc member is comprised of two identically arranged halves which are formed from an inexpensive, relatively thin material. The disc halves include central semicylindrical portions defining a hub for receiving the stem and a flange defining a continuous, peripheral groove including diametrically opposed annular portions which encircle the stem. Disposed in the disc groove is a unitary, resilient sealing ring including annular sections which engage flattened portions of the bore to provide stem seals throughout all positions of the disc and semicircular portions which sealingly engage the bore when the disc member is in a closed position. The disc halves and sealing ring can be assembled together before installation of the disc member into the valve body bore or assembled together inside the valve body bore.

United States Patent [191 Robinson [451 Sept. 2, 1975 BUTTERFLY VALVEDaniel T. Robinson, West Allis, Wis.

[73] Assignee: Milwaukee Valve Company, Inc.,

Milwaukee, Wis.

22 Filed: Sept. 27, 1973 21 Appl. No.: 401,343

[75] Inventor:

Primary Examiner-Charles J. Myhre Assistant Examiner-4m S. Lazarus [5 7ABSTRACT The butterfly valve includes a body having a generallycylindrical bore defining a flow passage and a generally circular,stem-mounted disc member rotatably mounted within the bore for shuttingoff flow. The disc member is comprised of two identically arrangedhalves which are formed from an inexpensive, relatively thin material.The disc halves include central semicylindrical portions defining a hubfor receiving the stem and a flange defining a continuous, peripheralgroove including diametrically opposed annular portions which encirclethe stem. Disposed in the disc groove is a unitary, resilient sealingring including annular sections which engage flattened portions of thebore to provide stem seals throughout all positions of the disc andsemicircular portions which sealingly engage the bore when the discmember is in a closed position.

The disc halves and sealing ring can be assembled together beforeinstallation of the disc member into the valve body bore or assembledtogether inside the valve body bore.

13 Claims, 9 Drawing Figures BUTTERFLY VALVE BACKGROUND OF THE INVENTIONThis invention relates to a butterfly valve and a method for makingsame.

The butterfly valve typically includes a circular disc member which ispivotally mounted inside a cylindrical bore defining a flow passage. Thedisc is mounted to turn on an axis extending transversely of the flowpassage between an open position where its plane is generally coincidentwith the direction of flow and a closed position where its plane is at aright angle to the direction of flow. In a closed position a fluid sealis provided either by the outer periphery of the disc sealingly engaginga sealing member provided in the interior of the valve housing or bysealing member carried on the pe riphery of the disc sealingly engaginga seating surface on the interior of the housing.

Discs used in prior art valves usually are machined or die cast parts.Consequently, butterfly valves have not been widely used in systemsrequiring a small shutofi' valve, e.g., in the /2 to 2 inch range,because the relatively massive disc adversely affects the flowcharacteristics of the valve. Also, machined or die cast discs representa significant portion of the total fabrication and assembly costs formost butterfly valves.

SUMMARY OF THE INVENTION An object of this invention is to provide abutterfly valve capable of being used in relatively small flow sys tems.

Another object of this invention is to provide a simply arrangedbutterfly valve which can be conveniently fabricated and assembled byinexpensive techniques.

A further object of this invention is to provide a butterfly valvehaving a disc which can be fabricated from relatively thin, inexpensivematerials.

A still further object of this invention is to provide a simple,inexpensive method for fabricating and assembling a butterfly valve.

Other objects, aspects and advantages of this invention will becomeapparent upon reviewing the following detailed description, the drawingand the appended claims.

The butterfly valve provided by the invention includes a body having agenerally cylindrical bore which defines a flow passage through thebody, a stem rotatably journaled in the body and extending transverselyof the flow passage, and a generally circular disc member mounted on thestem for rotatable movement within the bore between an open position anda closed position to shutoff flow. The disc member is composed of a pairof sections or halves which are formed from a relatively thin materialand are mounted in facing relationship on the stem. The disc halves arepreferably identically arranged and include portions which cooperate todefine a hub for receiving the stem and peripheral flanges whichcooperate to define a peripheral groove for receiving and positivelyretaining a resilient sealing ring.

The sealing ring preferably is a unitary member including opposedannular sections which encircle the stem to provide stem sealsthroughout all positions of the disc member and semicircular sectionswhich sealingly engage the bore when the disc member is in the closedposition.

The disc halves and sealing ring can be assembled together beforeinstallation of the disc member into the valve body bore or assembledtogether inside the valve body bore. In the latter case, the sealingring and stem preferably are first installed inside the bore and thedisc halves then installed in facing relationship through the oppositeends of the bore. After the hub-defining portions of the disc halves aremoved into engagement with the stem and the flanges. of the disc halvesare moved into compressive engagement with the sealing ring, the dischalves are fastened to the stem, such as by spot welding.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a side view of a butterflyvalve, partially sectioned, embodying various of the features of theinvention.

FIG. 2 is an enlarged, exploded view of the valve illustrated in FIG. 1.

FIG. 3 is a fragmentary, end view of the valve illustrated in FIG. 1.

FIG. 4 is a sectional view taken along the line designated 4-4 in FIG.2.

FIG. 5 is a sectional view taken along the line designated 55 in FIG. 4.

FIGS. 6, 7 and 8 are fragmentary, cross sectional views of the valveillustrated in FIG. 1 shown at various stages of assembly.

FIG. 9 is a fragmentary, sectioned view of an alternate arrangement forthe valve illustrated in FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Before explaining the inventionin detail, it is to be understood that the invention is not limited inits application to the details of construction and the arrangements ofthe components set forth in the following description or illustrated inthe drawing. The invention is capable of other embodiments and of beingpracticed and carried out in various Ways. Also, it is to be understoodthat the phraseology and terminology employed herein is for the purposesof description and should not be regarded as limiting.

Illustrated in the drawing is a butterfly valve 10 including a body 12having a generally cylindrical bore 14 which defines a flow passageextending substantially straight through body 12. The opposite ends ofvalve body 12 can be threaded as shown or otherwise formed in aconventional manner for connection with associated plumbing.

A generally circular disc member 16 is mounted on a shaft or stem 18 forrotatable movement within bore 14. Disc member 16 has a diametricallyextending, enlarged central portion 20 defining a hub 22 for receivingstem 18 and opposed annular bosses 24 surrounding hub 22 (See FIGS. 1and 7).. Disc member 16 is secured to stem 18 by a suitable means, suchas by spot welding.

Stem 18 includes an inner end which is rotatably received, i.e.,journaled, in a recess 30 provided in valve body 12 and an outer endwhich rotatably extends through an aperture 34 provided in valve body 12coaxially with recess 30. A suitable actuator means, such as a handle42, is fastened to the outer end of stem 18, such by a nut 44, forrotating disc member 16 about the transverse pivot axis provided by stem18 between a closed or flow shutoff position where the plane of discmember 16 is generally at a right angle to the direction of flow (SeeFIG. 3) and an open position where the plane of the disc issubstantially aligned with the direction of flow (See FIG. 1).Rotational movement of disc member 16 at the fully opened and fullyclosed positions is limited by a stop lug 46 provided on the exterior ofvalve body 12 and which is engaged by an inturned flange 48 located onthe end of handle 42.

Valve bore 14 includes diametrically opposed, flattened wall portions 50in thevicinities surrounding recess 30 and aperture 34 and diametricallyopposed, generally semicircular wall portions 52 extendingcircumferentially between flattened portions 50 (See FIGS. 1 and 3).Bore portions 50 and 52 serve as seating surfaces as explained in moredetail below. The outer surface of each disc boss 24 is flat and isspaced inwardly a small distance from the corresponding flattened boreportion 50. The circular portions of disc member 16 have an outer radiuswhich is slightly less than the radius of the corresponding boresemicircular portion 52.

For receiving a sealing ring 54, disc member 16 has a continuousperipheral recess or groove including opposed annular portions 56 (SeeFIG. 7) which are located in bosses 24 and encircle stem hub 22 andopposed generally semicircular portions 58 (See FIGS. 7 and 8) whichextend between bosses 24 and connect annular portions 56. semicirculargroove portions 58 have a dove tailed or generally inverted, V-shapedcross section.

Sealing ring 54 is formed as a unitary assembly and includes annularsections 60 which fit into annular portions 56 of the disc groove andsemicircular sections 62 which connect annular sections 60 and fit intosemicircular portions 58 of the disc groove. Sealing ring 54 is madefrom a relatively resilient material suitable for the type service inwhich the valve is to be used, such as natural rubber, syntheticelastomers, fluorocarbonmaterials and the like. Annular sections 60 ofsealing ring 54 have a generally ellipitical cross section (See FIG. andsemicircular sections 62 of sealing ring 54 have a generally trapezoidalcross section (See FIG. 4).

In order to provide an interference fit between annular sections 60 ofsealing ring 54 and flattened bore portions 50, each annular sealingring section 60 extends outwardly from annular groove portion 56 by adimension which exceeds the clearance between the outer surface of boss24 and the flattened bore portion 50. This interference fit, which ismaintained throughout all positions of disc member 16, causes annularsections 60 of sealing ring 54 to be compressed radially into sealingengagement with stem 18 (See FIG. 7). Thus, the annular portions of thesealing'ring provide fluid tight stem seals at all positions of discmember 16.

Semicircular sections 62 of sealing ring 54 extend. outwardly beyond theouter periphery of disc member 16 and sealingly engage the correspondingsemicircular bore portions 52 when disc member 16 is in the closedposition (See FIGS. 3, 7 and 8). The dove-tailed or inverted, V-shapedcross section of disc groove portions 58-and the trapezoidal shapedcross section of sealing ring sections 62 cooperate to provide apositive restraint against the sealing ring being twisted or beingdislodged from the disc groove by high velocity flow during valveoperation.

Any leakage past the stem seal into recess 30 might cause a build up ofpressure behind the inner end of stem 18 tending to force the stem outof valve body 12. This potential problem can be eliminated by ventingrecess 30 to the atmosphere, such as by extending recess 30 completelythrough the valve body or by providing a small vent port (not shown)which communicates with recess 30.

In the embodiment illustrated in FIGS. l3, the torque required to rotatedisc member 16 between a fully open position and a nearly closedposition is controlled by the surface area contact and the degree ofinterference between section 60 of sealing ring 54 and the flattenedbore portions 50. This torque can be quite low, particularly when thesealing ring 54 is made from a material having a relatively lowcoefficient of friction. A low torque can be quite desirable for someapplications particularly when the valve is manually operated and usedsolely for shutoff purposes. However, in other applications, such aswhere the valve is used to meter flow and the disc member is disposed atan intermediate position, the frictional drag provided by the annularsealing ring sections may not be sufficient to prevent the disc memberfrom being rotated by fluid forces.

To eliminate this problem, means can be provided for selectively varyingthe rotational drag on the stem. While various arrangements can be used,in the alternate embodiment illustrated in FIG. 9, valve body 12 isprovided with an annular recess 59 which is coaxial with stem-receivingaperture 34 and receives a packing material 61. The packing material 61is compressed into engagement with the outer surface of stem 18 by agland nut 63 threaded'into recess 59. The frictional drag imposed onstem 18 by packing material 61 is adjusted by tightening gland nut 63(to increase drag) or loosening gland nut 63 (to decrease drag). Packingmaterial 61 can be the same general type commonly used for valve glands,except it has a high coefficient of friction in order to be capable ofimposing a relatively high rotational drag on stem 18. For example, thepacking material can be a braided asbestos impregnated with a materialhaving a relatively high coefficient of friction. Thus, in addition toacting as a drag producing means for stem 18, the packing material alsoserves as a secondary stern seal.

In order to reduce fabrication and assembly costs and to minimize theflow restriction produced by the disc member, disc member 16 isconstructed from two separate halves 64 (See FIG. 1). Disc halves 64 aremirror images, and preferably identically arranged, with each includinga generally semicylindrical central portion 66 and a peripheral flange68 which, upon face-to-face assembly of the two disc halves, define stemhub 22 and groove portions 56 and 58, respectively. As best shown inFIG. 8, the peripheral flange 68 on each disc half 64 lies away from aplane coincident with the flat confronting or inner face 69.

With this arrangement, disc halves 64 can be conveniently andeconomically fabricated from a relatively rigid, inexpensive thin sheetmaterial, such as metal, plastic or the like. While other materials andforming techniques can be used, disc halves 64 are preferably formedfrom a thin sheet (e.g., l/32 inch thick) of stainless steel (e.g.,7,000 series) by a conventional metal stamping process.

Disc member 16 and sealing ring 54 can be assembled either outside orinside valve body 12. In the former case, disc halves 64 are firstsecured together in face-to-face relationship with sealing ring 54disposed between flanges 68. After the disc member/sealing ring assemblyhas been installed inside valve body bore 141 with bosses 24 and annularsealing rings sections 60 axially aligned with the corresponding recess30 and aperture 34, stem 18 is installed, disc member 16 fastened tostem 18, and the remaining assembly steps are completed in the usualmanner.

Referring to FIGS. 6-8, an exemplary method for as sembling valve in thelatter manner will be described. Sealing ring 54 is first installed intovalve body bore 14, such as with a suitably arranged holding fixture(not shown), with annular sections 60 in coaxial alignment with recessand aperture 34- and semicircular sections 62 located in a positioncorresponding to a closed position of disc member 16 (See FIG. 6). Stem18 is thereafter inserted through aperture 34, passed through theopenings of both sealing ring sections and the inner end moved intorecess 30. The two disc halves 64 are then installed.

Suitable means are provided for holding disc halves 64 so they can beinstalled, in facing relation, through opposite ends of bore 14. Whileother arrangements can be used, in the specific method illustrated, suchmeans includes separate fixtures and 72, each having a hollow jig 74adapted to carry a disc half 64 on the outer end and a centrallyapertured clamping bracket 76 (See FIG. 7). The disc halves 64, carriedon fixtures 70 and 72, are moved into place inside valve bore 14 withsemi-cylindrical portions 66 adjacent to the opposite sides of stem 18and flanges 68 adjacent to the opposite edges of sealing ring 54.

Fixtures 70 and 72 are illustrated in a horizontal position in FIGS. 7and 8, in which case a suitable means, such as a magnet or the like, isused to temporarily retain disc halves 64 in place on the outer endsofjigs 74. In actual practice, it may be more desirable for the fixtures to be in a vertical position during installation of the dischalves. When the fixtures are in this position, the valve body is moveddown over the jig of a first fixture carrying one of the disc halves,rotated 180 with the first fixture being held in place, and then moveddown the jig of a second fixture carrying the other disc half.

After the disc halves have been installed, fixtures 70 and 72 areclamped together, such as by tightening a plurality of bolts 78 (oneshown) connecting clamping brackets 76. The resultant clamping action ondisc halves 64 moves semicylindrical portions 66 into engagement withstem 18 and causes flanges 68 to com press both the annular sections 60and the semicircular sections 62 of sealing ring 54.

semicylindrical portions 66 of disc halves 64- are then secured to stem18 in a suitable manner. While other techniques can be employed, in thespecific method illustrated, a pair of oppositely disposed weldingelectrodes 80 of a conventional spot welding machine (not shown) areinserted through the openings of fixtures 70 and 72 and moved intoengagement with the outer surfaces of semicylindrical portions 66. Thewelding machine is energized to spot weld each disc half 64 to stem 18at one or more locations.

If desired, other fastening means, such as rivets, bolts, pins and thelike, can be used to fasten the disc member to the stem. When made froma plastic mate rial, the disc member can be secured to a metallic stemby an adhesive, such as an epoxy material which is suitable for thatpurpose. When both the disc halves and the stem are made from athermoplastic material, the disc member can be fused to the stem by asuitable thermoplastic welding technique.

From the above detailed description, it can be seen that a butterflyvalve arranged in accordance with the invention has many distinctadvantages. The disc member is formed from two halves which can beidentically arranged and fabricated from relatively thin materials. Theresultant thinness of the disc member minimizes the flow restrictionproduced thereby, permitting the butterfly valve to be usedadvantageously in small flow systems, e.g., t0 2 inches. Instead ofrequiring expensive machining or die casting operations, the disc halvescan be fabricated by conventional inexpensive forming techniques whichreduces the overall fabrication and assembly costs of the valve. Thesealing ring does not have to be stretched for mounting into the discmember groove; thus, the sealing ring can be made from seal materialswhich ordinarily tend to become permanently deformed by stretching. Thesealing ring is positively restrained against. being dislodged from thedisc member groove by high velocity flow. Moreover, the disc member canbe conveniently assembled either outside or inside the valve body bore.

I claim:

1. A butterfly valve comprising a body having a generally cylindricalbore defining a flow passage therethrough;

a stem rotatably mounted in said body and extending transversely of saidflow passage;

a generally circular disc member carrying a resilient sealing ring onthe outer periphery and supported on said stem for rotatable movementbetween an open position and a closed position where said sealing ringsealingly engages said bore to shut off flow through said flow passage;

said disc member including a pair of relatively thin circular dischalves disposed in confronting faceto-face relationship, each of saiddisc halves being formed with a diametrically extending semicylindricalgroove portion cooperating with the corresponding semicylindricalportion of the other of said disc halves to define a hub receiving saidstem and each of said disc halves also being formed with a peripheralflange lying away from a plan coinci' dent with the confronting face andcooperating with the corresponding peripheral flange on the other ofsaid disc halves to define a peripheral groove therebetween receivingsaid sealing ring, said cooperating flange compressingly engaging saidsealing ring to positively restrain said sealing ring from being twistedor dislodged from said peripheral groove during opening and closing ofsaid disc member; and

means for securing both of said disc halves to said stem.

2. A butterfly valve according to claim 1 wherein said bore includesdiametrically opposed flattened wall portions in the vicinity of saidstem and diametrically opposed, generally semicircular wall portionsjoining said flattened wall portions;

'said, peripheral groove includes a pair of diametrically opposedannular portions respectively encircling the opposite ends of said stemhub and a pair of diametrically opposed generally semicircular portionsextending circumferentially between corresponding of said annularportions;

said sealing ring is a unitary member and includes a pair of opposedannular sections disposed in corresponding of said annular grooveportions, said annular sealing ring sections being in an interferencefit with the corresponding flattened wall portions of said borethroughout all positions of said disc member; and

said sealing ring further includes a pair of opposed, generallysemicircular sections connecting said an nular sections and disposed incorresponding of said semicircular groove portions, said semicircularsealing ring sections sealingly engaging corresponding semicircular wallportions of said bore when said disc member is in a closed position.

3. A butterfly valve according to claim 2 wherein the portions of saidperipheral flanges defining said semicircular portions of saidperipheral groove converge radially outwardly; and

said semicircular portions of said sealing ring have a generallytrapezoidal cross section and include a smaller outermost portionextending radially beyond the peripheries of the corresponding discflanges.

4. A butterfly valve according to claim 1 wherein said securing meanscomprises a welded connection between each of said semicylindricalportions and said stem on opposite sides of said stem.

5. A butterfly valve according to claim 4 wherein each of said dischalves is formed from a thin sheet of stainless steel.

6. A butterfly valveaccording to claim 5 wherein said disc halves areformed by stamping.

7. A butterfly valve according to claim 6 wherein said disc halves aremirror images of each other.

8. A butterfly valve according to claim 1 including means forselectively imposing sufficient rotational drag on said stem to retainsaid disc member at positions intermediate the open and closed positionswhereby said disc member can throttle flow through said flow passage andyet can be moved to the open and closed positions.

9. A butterfly valve according to claim 8 wherein said means forselectively imposing rotational drag on said stem comprises an annularrecess in said body and encircling one end portion of said stem;

a material having a high coefficient of friction dis-- posed in saidrecess and compressingly engaging said stem; and

a gland nut threadably received in said body and disposed in compressiveengagement with said material for selectively adjusting the compressionof said material with said stem.

10. A butterfly valve comprising a body having a generally cylindricalbore defining a flow passage therethrough, said bore includingdiametrically opposed flattened wall portions and diametrically opposedgenerally semicircular wall portions joining said flattened wallportions;

a rotable stem extending between said flattened wall portions andperpendicularly to the direction of flow;

a generally circular disc member carrying a resilient sealing ring onthe outer periphery and supported on said stem for rotatable movementbetween an open position generally aligned with the direction of flowand a closed position generally perpendicular to the direction of flow;

said sealing ring being a unitary member including a pair of opposedannular sections encircling corresponding opposite ends of said stem anddisposed in an interference fit with corresponding flattened wallportions of said bore throughout all positions of said disc member toform a seal about said stem;

said sealing ring further including a pair of opposed,

generally semicircular sections connecting said annular sections andsealingly engaging corresponding semicircular wall portions of said borewhen said disc member is in a closed position;

said disc member including a pair of relatively thin circular dischalves disposed in confronting, faceto-face relationship, each of saiddisc halves being formed with a diametrically extending, semicylindricalgroove portion cooperating with the semicylindrical groove portion ofthe other of said disc halves to define a hub receiving said stem andeach of said disc halves further being formed with a peripheral flangelying away from a plane coincident with the confronting face andcooperating with the corresponding peripheral flange on the other ofsaid disc halves to define a peripheral groove therebetween receivingsaid sealing ring;

said cooperating disc flanges including first portions defining a pairof diametrically opposed annular groove portions encircling the oppositeends of said stern hub and receiving corresponding annular portions ofsaid sealing ring;

said cooperating disc flanges including second portions defining a pairof diametrically opposed, generally semicircular groove portionsextending circumferentially between corresponding annular grooveportions and receiving corresponding semi circular sealing ringsections, said second flange portions compressively engaging saidsemicircular sealing ring portions to positively restrain saidsemicircular sealing portions from being twisted or dislodged from saidperipheral groove during opening and closing of said disc member; and

each of said disc halves being secured to said stem by a weldedconnection between each of said semicylindrical portions and said stemon opposite sides of said stem.

11. A butterfly valve according to claim 10 wherein said disc halves aremirror images of each other and are formed as stampings from a thinsheet of stainless steel.

12. A butterfly valve according to claim 11 wherein said second portionsof said peripheral flange converge radially outwardly; and

said sealing ring semicircular portions have a generally trapezoidalcross section and include a smaller outermost portion extending radiallybeyond the peripheries of the corresponding disc flanges.

13. A butterfly valve according to claim 12 including an annular recessin said body and encircling one end portion of said stem;

a material having a high coefficient of friction disposed in said recessand compressingly engaging said stem; and

a gland nut threadably received in said body and disposed in compressiveengagement with said material for selectively adjusting the compressionof said material with said stem and selectively imposing sufficientrotational drag on said stem to retain said disc member at positionsintermediate the open and closed positions whereby said disc member canthrottle flow through said flow passage.

v UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION PATENT NO.3,902,697 DATED September 2, 1975 INVENTOR(S) Daniel T. Robinson It iscertified that error appears in the above-identified patent and thatsaid Letters Patent are hereby corrected as shown below:

Claim 1, line 25, delete "flange" and substitute therefor flangesgignfid and Sealed this twenty'fifth D y f November 1975 Attest:

RUTH C. MASON Commissioner oj'Parents and Trademarks

1. A butterfly valve comprising a body having a generally cylindrical bore defining a flow passage therethrough; a stem rotatably mounted in said body and extending transversely of said flow passage; a generally circular disc member carrying a resilient sealing ring on the outer periphery and supported on said stem for rotatablE movement between an open position and a closed position where said sealing ring sealingly engages said bore to shut off flow through said flow passage; said disc member including a pair of relatively thin circular disc halves disposed in confronting face-to-face relationship, each of said disc halves being formed with a diametrically extending semicylindrical groove portion cooperating with the corresponding semicylindrical portion of the other of said disc halves to define a hub receiving said stem and each of said disc halves also being formed with a peripheral flange lying away from a plan coincident with the confronting face and cooperating with the corresponding peripheral flange on the other of said disc halves to define a peripheral groove therebetween receiving said sealing ring, said cooperating flange compressingly engaging said sealing ring to positively restrain said sealing ring from being twisted or dislodged from said peripheral groove during opening and closing of said disc member; and means for securing both of said disc halves to said stem.
 2. A butterfly valve according to claim 1 wherein said bore includes diametrically opposed flattened wall portions in the vicinity of said stem and diametrically opposed, generally semicircular wall portions joining said flattened wall portions; said peripheral groove includes a pair of diametrically opposed annular portions respectively encircling the opposite ends of said stem hub and a pair of diametrically opposed generally semicircular portions extending circumferentially between corresponding of said annular portions; said sealing ring is a unitary member and includes a pair of opposed annular sections disposed in corresponding of said annular groove portions, said annular sealing ring sections being in an interference fit with the corresponding flattened wall portions of said bore throughout all positions of said disc member; and said sealing ring further includes a pair of opposed, generally semicircular sections connecting said annular sections and disposed in corresponding of said semicircular groove portions, said semicircular sealing ring sections sealingly engaging corresponding semicircular wall portions of said bore when said disc member is in a closed position.
 3. A butterfly valve according to claim 2 wherein the portions of said peripheral flanges defining said semicircular portions of said peripheral groove converge radially outwardly; and said semicircular portions of said sealing ring have a generally trapezoidal cross section and include a smaller outermost portion extending radially beyond the peripheries of the corresponding disc flanges.
 4. A butterfly valve according to claim 1 wherein said securing means comprises a welded connection between each of said semicylindrical portions and said stem on opposite sides of said stem.
 5. A butterfly valve according to claim 4 wherein each of said disc halves is formed from a thin sheet of stainless steel.
 6. A butterfly valve according to claim 5 wherein said disc halves are formed by stamping.
 7. A butterfly valve according to claim 6 wherein said disc halves are mirror images of each other.
 8. A butterfly valve according to claim 1 including means for selectively imposing sufficient rotational drag on said stem to retain said disc member at positions intermediate the open and closed positions whereby said disc member can throttle flow through said flow passage and yet can be moved to the open and closed positions.
 9. A butterfly valve according to claim 8 wherein said means for selectively imposing rotational drag on said stem comprises an annular recess in said body and encircling one end portion of said stem; a material having a high coefficient of friction disposed in said recess and compressingly engaging said stem; and a gland nut threadably received in said body and disposed in compressive engagement with said material for selectively adjusting the compression of saId material with said stem.
 10. A butterfly valve comprising a body having a generally cylindrical bore defining a flow passage therethrough, said bore including diametrically opposed flattened wall portions and diametrically opposed generally semicircular wall portions joining said flattened wall portions; a rotable stem extending between said flattened wall portions and perpendicularly to the direction of flow; a generally circular disc member carrying a resilient sealing ring on the outer periphery and supported on said stem for rotatable movement between an open position generally aligned with the direction of flow and a closed position generally perpendicular to the direction of flow; said sealing ring being a unitary member including a pair of opposed annular sections encircling corresponding opposite ends of said stem and disposed in an interference fit with corresponding flattened wall portions of said bore throughout all positions of said disc member to form a seal about said stem; said sealing ring further including a pair of opposed, generally semicircular sections connecting said annular sections and sealingly engaging corresponding semicircular wall portions of said bore when said disc member is in a closed position; said disc member including a pair of relatively thin circular disc halves disposed in confronting, face-to-face relationship, each of said disc halves being formed with a diametrically extending, semicylindrical groove portion cooperating with the semicylindrical groove portion of the other of said disc halves to define a hub receiving said stem and each of said disc halves further being formed with a peripheral flange lying away from a plane coincident with the confronting face and cooperating with the corresponding peripheral flange on the other of said disc halves to define a peripheral groove therebetween receiving said sealing ring; said cooperating disc flanges including first portions defining a pair of diametrically opposed annular groove portions encircling the opposite ends of said stem hub and receiving corresponding annular portions of said sealing ring; said cooperating disc flanges including second portions defining a pair of diametrically opposed, generally semicircular groove portions extending circumferentially between corresponding annular groove portions and receiving corresponding semicircular sealing ring sections, said second flange portions compressively engaging said semicircular sealing ring portions to positively restrain said semicircular sealing portions from being twisted or dislodged from said peripheral groove during opening and closing of said disc member; and each of said disc halves being secured to said stem by a welded connection between each of said semicylindrical portions and said stem on opposite sides of said stem.
 11. A butterfly valve according to claim 10 wherein said disc halves are mirror images of each other and are formed as stampings from a thin sheet of stainless steel.
 12. A butterfly valve according to claim 11 wherein said second portions of said peripheral flange converge radially outwardly; and said sealing ring semicircular portions have a generally trapezoidal cross section and include a smaller outermost portion extending radially beyond the peripheries of the corresponding disc flanges.
 13. A butterfly valve according to claim 12 including an annular recess in said body and encircling one end portion of said stem; a material having a high coefficient of friction disposed in said recess and compressingly engaging said stem; and a gland nut threadably received in said body and disposed in compressive engagement with said material for selectively adjusting the compression of said material with said stem and selectively imposing sufficient rotational drag on said stem to retain said disc member at positions intermediate the open and closed positions whereby said disc member can throttle flow through said flow Passage. 